DESCRIPTION: This is a revision of a proposal that was previously submitted in response to the request for applications on "Methodological Studies to Enhance Demographic Research". It was previously reviewed by a special review committee. The aim of the proposal is to extend the existing body of theoretical research on issues of fecundability, sterility and postpartum infecundity within noncontracepting populations by improving upon current theoretical models and by applying these models to two historical data sets. The two data sets that will be used in the proposed study are: 1) birth histories of the French Canadian population of women married between 1660 and 1729 and 2) the Hutterite population, living in the mid-20th century, in North America. These populations have the advantage of not suffering from diseases that might result in impaired reproductive health, and of not using any deliberate fertility control. Therefore observed age patterns of fecundability and sterility for these two populations should be largely related to natural age-related biological factors. Improved models of the level and age pattern of sterility, postpartum infecundity and fecundability that are multistate, that use continuous- time hazard models and that include both sterility and postpartum infecundity simultaneously, will be developed first theoretically, and then empirical applications will be carried out. The proposed project is an extension of recent work done by the principal investigator and Dr. Vaupel, which developed a new approach to modelling in this area, and which was recently applied for the first time to empirical data in their 1993 article in Demography.